On the afternoon of Oct. 1, 2013, a tall, slender, shaggy-haired man left his house on 15th Avenue in San Francisco. His two housemates knew him only as a quiet currency trader named Josh Terrey. His real name was Ross Ulbricht. He was 29 and had no police record. Dressed in jeans and a red T-shirt, Ulbricht headed to the Glen Park branch of the public library, where he made his way to the science-fiction section and logged on to his laptop  he was using the free wi-fi. Several FBI agents dressed in plainclothes converged on him, pushed him up against a window, then escorted him from the building.

The FBI believes Ulbricht is a criminal known online as the Dread Pirate Roberts, a reference to the book and movie The Princess Bride. The Dread Pirate Roberts was the owner and administrator of Silk Road, a wildly successful online bazaar where people bought and sold illegal goods  primarily drugs but also fake IDs, fireworks and hacking software. They could do this without getting caught because Silk Road was located in a little-known region of the Internet called the Deep Web.

The Deep Web is a specific branch of the Internet that's distinguished by that increasingly rare commodity: complete anonymity. As such, it is a vital tool for intelligence agents, law enforcement, political dissidents and anybody who needs or wants to conduct their online affairs in private  which is, increasingly, everybody.

But some prosecutors and government agencies think that Silk Road was just the thin edge of the wedge and that the Deep Web is a potential nightmare, an electronic haven for thieves, child pornographers, human traffickers, forgers, assassins and peddlers of state secrets and loose nukes.